1. Field of the Invention
The present invention relates to a bidirectional shift register capable of shifting a pulse in both forward and backward directions and to a drive circuit and a flat display device, both of which use the bidirectional shift register.
2. Description of the Related Art
In recent years, a flat display device typified by a liquid crystal display device has been used in various kinds of equipment because the device is thin, lightweight and low in power consumption. In order to realize further thinning, weight saving and reduction in cost, a technology of configuring a drive circuit by use of polysilicon thin-film transistors with higher electron mobility than conventional amorphous silicon thin-film transistors and forming this drive circuit on a glass array substrate has been recently established.
Generally, a liquid crystal display device includes an array substrate, a counter substrate disposed opposite thereto and a liquid crystal layer disposed in a gap between the both substrates. On the array substrate, a plurality of scan lines and a plurality of signal lines are disposed so as to intersect with each other. At respective intersections of these scan lines and signal lines, switching elements and pixel electrodes are disposed. Moreover, on the array substrate, a signal line drive circuit that drives the signal lines and a scan line drive circuit that drives the scan lines are disposed as drive circuits. The signal line drive circuit includes a horizontal shift register that outputs a pulse to each of the signal lines by shifting a phase of the pulse. The scan line drive circuit includes a vertical shift register that outputs a pulse to each of the scan lines by shifting a phase of the pulse. On the counter substrate, counter electrodes electrically corresponding to the pixel electrodes are disposed.
In general, for the horizontal shift register and the vertical shift register, a three-phase shift register having a plurality of series-connected shift registers with the same configuration is used. In this three-phase shift register, by use of three clock signals having different phases from each other, a shift register shifts a phase of a pulse before transmitting the pulse to other shift register in a subsequent stage.
However, in the case of transmitting the pulse in a backward direction, an output signal of each shift register cannot be obtained at a sufficient potential level with the same circuit configuration as that in the case of the forward direction. Moreover, there occurs a variation in the potential level of the output signal between the forward direction and the backward direction. Thus, variation was a factor deteriorating display quality.
It is an object of the present invention to provide a bidirectional shift register that prevents variation in the potential level of the output signal between the forward direction and the backward direction in terms of the direction of transmitting the pulse.
It is another object of the present invention to provide a drive circuit and a flat display device, both of which use the foregoing bidirectional shift register.